api.c: Add function to build tasks/procs path

api.c: Add function to build tasks/procs path

Replace the inline logic in multiple functions that concatenates
the path and cgroup v1 tasks file with a function -
cgroup_build_tasks_procs_path() - that works for both cgroup v1
and v2.  In the v1 case, it appends tasks, and in the v2 case, it
appends cgroup.procs.

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>